Hitherto, a sea water cooling apparatus for a marine diesel engine with a supercharger for use in a power boat or the like has been structured as shown in FIGS. 9 or 10. A sea water pump 4 has, on the inlet side thereof, an inlet conduit 6 the opening of which is positioned below the sea level 11 and, on the outlet side thereof, it has outlet conduits 7a and 7b through which cooling sea water passes and in which an after cooler 3 for the supercharger, an oil cooler 2 for the engine and an engine 1 are disposed. A thermostat 5 is disposed between the outlet conduits 7a and 7b, the thermostat 5 and the inlet conduit 6 of the sea water pump 4 being connected to each other by a bypass conduit 8. A fuel injection pump (omitted from illustration) is provided with an overflow valve for equalizing the temperature and the pressure of the fuel in such a manner that it is connected to a fuel tank 13 by an overflow conduit 15. When the engine 1 is operated under light load and thereby shows low temperature, the thermostat 5 is completely closed and cooling sea water supplied from the sea water pump 4 is warmed up by the engine 1 before it circulates the outlet conduit 7a, the thermostat 5 and the bypass conduit 8 so that the warming up operation is performed. When the load of the engine 1 is enlarged and thereby the temperature of cooling sea water is raised to a predetermined level, the thermostat 5 is completely opened so that cooling sea water cools the after cooler 3 for the supercharger, the oil cooler 2 for the engine and the engine 1 before it is discharged to the sea level 11 via the outlet conduit 7a, the thermostat 5 and the outlet conduit 7b.
However, in the sea water cooling apparatus for a marine diesel engine with a supercharger constituted in such a manner that the after cooler 3, the oil cooler 2 and the engine 1 are disposed in series, a problem arises in that a satisfactory large output cannot be obtained because the temperature of air at the outlet port of the after cooler is undesirably raised by the thermostat in a case where a rapid acceleration operation is performed after the warming-up operation has been performed or in a case of an operation under large load.
Another problem arises in that the temperature of fuel in the fuel tank is undesirably raised by fuel returned from the overflow valve, and, in particular, that the output is undesirably reduced if the quantity of fuel in the fuel tank is reduced.